using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        string input = "Mon 06:23-Fri 18:00";

        var interval = input.Split("-");

        var start = new InputInterval(GetDayIndex(interval[0].Split(" ")[0]), TimeSpan.Parse(interval[0].Split(" ")[1]));

        var end = new InputInterval(GetDayIndex(interval[1].Split(" ")[0]), TimeSpan.Parse(interval[1].Split(" ")[1]));

        Console.WriteLine(start.DayOfWeek.ToString() + ' '+ start.Time);

        Console.WriteLine(end.DayOfWeek.ToString() + ' '+ end.Time);

        Console.WriteLine(GetInterval(DateTime.Now, start));

        Console.WriteLine(GetInterval(DateTime.Now, end));

    }

    private static int GetDayIndex(string dayNameAbbreviation)

    {

        return (Array.IndexOf(CultureInfo.InvariantCulture.DateTimeFormat.AbbreviatedDayNames, dayNameAbbreviation)+6)%7;

    }

    private static DateTime GetInterval(DateTime now, InputInterval input)

    {

        var d = new DateTime(now.Year, now.Month, now.Day, input.Time.Hours, input.Time.Minutes, 0);

        d = d.AddDays(input.DayOfWeek - ((int)now.DayOfWeek+6)%7);

        return d;

    }

}

​

public class InputInterval

{

    public int DayOfWeek {get;set;}

    public TimeSpan Time {get;set;}

    public InputInterval(int dow, TimeSpan t) 

    {

        DayOfWeek = dow;

        Time = t;

    }

}